1. Field of the Invention
This invention relates to an apparatus for limiting a radiation exposure field in an equipment for the radiation therapy or for the non-destructive inspection using radiations, and more particularly to an apparatus for accurately defining a radiation exposure field while preventing radiation leakage from a gap between radiation shielding members.
2. Description of the Prior Art
Conventionally, a radiation exposure field limiting apparatus is incorporated, for example, in a linear electron accelerator for the medical application in which radiations such as, for example, X-rays are generated.
An exemplary one of conventional linear electron accelerators for the medical application is schematically shown in FIG. 9. Referring to FIG. 9, the conventional linear electron accelerator shown includes a fixed frame 1, a rotatable frame 2 supported for rotation around a horizontal axis 6 on the fixed frame 1, and a medical table 5 on which a top plate 4 for supporting thereon a patient to undergo radiation therapy is supported. X-rays 8 are emitted from a radiation source 11 and irradiated along a radiation center axis 7 toward an intersecting point between the axis 6 of rotation of the rotatable frame 2 and the radiation center axis 7 of X-rays 8. Such intersecting point is the center of medical treatment at which the affected part of a patient to be treated is normally positioned and will be hereinafter called an iso-center. Meanwhile, a line 10 shown in FIGS. 11 and 12 which passes the iso-center 9 and extends orthogonally to the axis 6 of rotation and the X-ray radiation center axis 7 will be hereinafter called an exposure field center axis. Referring also to FIGS. 10 and 11, the size of an exposure field to be formed by X-rays 8 generated from the radiation source 11 is defined by a pair of radiation shielding devices 12 and 13 disposed along the X-ray radiation center axis 7 for delineating an exposure field in perpendicular directions to each other and each composed of a pair of radiation shielding members or blocks 12a and 12b or 13a and 13b. The radiation shielding blocks 12a, 12b, 13 a and 13b are made of a heavy metal such as lead.
Referring now to FIG. 10, an electron beam 50 emitted from an electron beam source not shown is accelerated by an accelerating tube 47 and then guided by a detecting electromagnet 49 in a beam duct 48 maintained in a vacuum condition so that it is introduced to the radiation source 11 at which X-rays 8 are generated in response to the electron beam 50. A primary collimator 43 for defining a maximum extent of X-rays 8 is disposed just below the radiation source 11, and a flattening filter 44 for making the distribution of X-rays 8 in an exposure field uniform is disposed just below the primary collimator 43. Further, a dose monitor 45 for monitoring an amount of X-rays 8 on the real time basis is disposed just below the flattening filter 44, and a mirror 46 is interposed between the dose monitor 45 and the radiation shielding blocks 12a and 12b. The mirror 46 is disposed such that visible rays of light from a light source 51 may be introduced to provide the same extent as X-rays 8 in order to permit visual observation of an X-ray exposure field. The light source 51 is disposed at a position equivalent to that of the radiation source 11 with respect to the mirror 46.
Referring also to FIG. 11, the dimension of an exposure field which is projected from the radiation source 11 or the light source 51 by way of the radiation shielding blocks 12a and 12b onto a plane including the iso-center 9 and extending orthogonally to the X-ray radiation center axis 7 (such plane will be hereinafter referred to as an iso-center plane) is represented by a capital letter "L", and the dimension of the exposure field which is projected similarly by way of the radiation shielding blocks 13a and 13b onto the iso-center plane is represented by another capital letter "W".
FIG. 12 shows such exposure field as viewed from the radiation source 11. Referring to FIG. 12, a rectangular area having sides of the dimensions L and W is denoted at 41, and the affected part (such as a tumor) to be medically treated in a body of a patient 3 is denoted at 42.
Referring to FIG. 13, each of the radiation shielding blocks 13a and 13b is shown formed from a plurality of radiation shielding members or parts 31a to 36a or 31b to 36b such that they may define an X-ray exposure area of a profile approximated to that of the affected part 42. Such radiation shielding parts 31a to 36a and 31b to 36b will be hereinafter called each a leaf, and here, the leaves 31a and 31b are center leaves which define an exposure area portion W1 along the exposure field center axis 10 while the other leaves are sequentially numbered toward the opposite outer sides from the center leaves 31a and 31b beginning with the number 32. While several leaves on the left-hand side in FIG. 13 of the center leaves 31a and 31b are not denoted by any reference character, they should be considered to be numbered similarly as 32a 36a and 32b to 36b. Accordingly, the radiation shielding blocks 13a and 13b include seven pairs of leaves in the arrangement shown in FIG. 13. Naturally, however, they may include any arbitrary plural number of pairs of leaves. A radiation shielding block including a plurality of leaves will be hereinafter referred to as a multi-leaf radiating shielding block.
Referring now to FIG. 14, there is shown an exemplary structure of such multi-leaf radiation shielding block as shown in FIG. 13, and each of the radiation shielding blocks 12a and 12b shown in FIG. 10 may be replaced by such multi-leaf radiation shielding block as shown in FIG. 14. Though not shown, the other radiation shielding block 12b or 13b includes leaves 31b to 36b corresponding to the leaves 31a to 36a. In the case of the multi-leaf radiation shielding block shown in FIG. 14, each of the leaves has a rectangular cross section.
FIG. 15 shows another exemplary structure of a multi-leaf radiation shielding block. The multi-leaf radiation shielding block shown in FIG. 15 includes a similar number of leaves which are numbered in a similar manner but have different sectional areas from those of the leaves shown in FIG. 14. In particular, each of the leaves has a sectional area of a generally trapezoidal shape as is provided by cutting a circular cone having the apex at the radiation source 11 along a generating line. In order to prevent X-rays from passing through a gap between each adjacent ones of the leaves, each of the leaves has a projection 39 formed thereon, and the projection 39 is fitted for sliding movement in a complementary recess formed in an opposing face of an outer adjacent one of the leaves.
FIG. 16 illustrates a principle of construction of each leaf. Referring to FIG. 16, reference numerals 61 and 62 denote generating lines on faces of two circular cones having the apexes commonly at the radiation source 11. Thus, the leaves shown in FIG. 15 are constituted if such a member as is indicated by a hatched portion between the generating lines 61 and 62 in FIG. 16 and similar members as are defined similarly by generating lines are replaced into the individual leaves shown in FIG. 15 and then a projection 39 is provided on each of the leaves while each of the leaves is machined to form a complementary recess in which the projection 39 of an inner adjacent one of the leaves is fitted.
FIGS. 17 and 18 show portions of an exposure field in the direction along L which are formed by such leaves. In particular, FIG. 17 shows, as an example, exposure field portions formed by the leaves 32a and 33a of the radiating shielding block shown in FIG. 14. The exposure field portion projected from the light source 51 is such as denoted by La where the leaf 32a is positioned farther from the axis 6 of rotation than the leaf 33a, but is such as denoted by Lb where the leaf 32a is positioned nearer to the axis 6 of rotation than the leaf 33a. Meanwhile, FIG. 18 shows exposure field portions formed by the leaves 32a and 33a of the radiating shielding block shown in FIG. 15. Similarly, where the leaf 32a is farther from the axis 6 of rotation than the leaf 33a, such an exposure field portion as denoted by Lc is formed, but where the leaf 32a is nearer than the leaf 33a, such another exposure field portion as denoted by Ld is formed.
Subsequently, operation of the linear electron accelerator will be described.
Referring back to FIG. 9, a patient 3 to undergo radiation therapy will first lie on the top plate 4. In order to permit X-rays 8 for the radiation therapy generated from the radiation source 11 to be irradiated from any position around the patient 3, the radiation source 11 can be circularly moved over an angular range greater than 360 degrees around the axis 6 of rotation by means of the rotatable frame 2. Further, the patient 3 can be moved in leftward and rightward directions and in forward and backward directions by means of the top plate 4. Since the top plate 4 can be moved also in upward and downward directions by means of the medical table 5, the affected part 42 of the patient 3 to be irradiated can be positioned in an area including the iso-center 9 to perform radiation therapy.
Referring now to FIGS. 10 and 11 which illustrate a manner of generating X-rays and determining an exposure field, an electron beam 50 is accelerated to a high energy, for example, to 3 to 20 MeV by the accelerating tube 47 and then runs in the beam duct 48 in a vacuum condition while it is deflected by the deflecting electromagnet 49 so that it may be directed toward the patient 3. Consequently, the electron beam 50 is introduced to the radiation source 11 which is made of a metal material which generates X-rays when it is irradiated by an electron beam. The radiation source 11 thus generates intense X-rays 8 in the advancing direction of the electron beam 50. While the X-rays 8 have a forward directivity, the primary collimator 43 is disposed in order to absorb unnecessary X-rays outside a required exposure field. Since the X-rays 8 are spread radially from the radiation source 11 which serves as a point radiation source, the primary collimator 43 is scooped out to form a hole having the profile of a truncated cone having the apex at the radiation source 11, and the X-rays 8 can pass only through the area of the hole. Generally, an exposure field defined by such truncated conical hole of the primary collimator 43 has a maximum dimension specified with the arrangement. Further, since X-rays generally have a directivity, the flattening filter 44 is disposed to make the distribution in intensity of the X-rays 48 uniform, and while such X-rays 48 continue to be generated, the intensity of the X-rays 48 is monitored on the real time basis by means of the dose monitor 45. The X-rays 8 are then stopped by the radiation shielding blocks 12a, 12b and 13a, 13b so that they may be irradiated upon the affected part 42 of the patient 3 in such an exposure field area 41 as seen in FIG. 12. In order to allow visual observation of such exposure field, the mirror 46 is disposed such that visible light which is emitted from the light source 41 disposed at the equivalent position to that of the radiation source 11 with respect to the mirror 46 is reflected by the mirror 46 so that an exposure field of the light may be produced on a face of the skin of the patient 3. Thus, the size of the exposure field, that is, the dimensions L and W can be confirmed from the exposure field of the visible light.
While the linear electron accelerator having a rectangular exposure field operates in such a manner as described above, since X-rays are irradiated also upon normal structure of the patient other than the affected part as seen in FIG. 12, there is the possibility of radiation hazard to such normal structure. Accordingly, a linear electron accelerator in recent years is constituted such that an exposure field is defined by radiation shielding blocks of such multi-leaf construction as shown FIG. 13 so as to assure protection of normal structure. Thus, the radiation shielding blocks 13a and 13b are replaced by those which are formed from leaves 31a to 36a and 31b to 36b, respectively, so that an exposure field in the W direction in FIG. 12 may have several dimensions W1, W2, . . . while the exposure field in the L direction is defined by the radiation shielding blocks 12a and 12b. Where such multi-leaf construction is employed, radiation therapy can be achieved with a higher degree of accuracy.
Such multi-leaf radiation shielding blocks as shown in FIG. 14 or 15 have been proposed and produced with an intention to obtain such an exposure field as shown in FIG. 13. In particular, in place of the radiation shielding blocks 13a and 13b, multi-leaf radiation shielding blocks are disposed at the locations of the radiation shielding blocks 13a and 13b which are normally positioned at substantially mid locations between the radiation source 11 and the iso-center 9. In the case of the arrangement shown in FIG. 14 in which a multi-leaf radiation shielding block is viewed from a similar point of view to that in FIG. 10, each leaf has a rectangular cross section. A radiation shielding block is constituted in most cases from an odd plural number of leaves because it is normally necessary to determine the dimension W1 of an exposure field portion along the exposure field center axis and consequently the radiation shielding block includes a center leaf and a pair or pairs of leaves on the opposite sides of the center leaf. In the case of the arrangement of FIG. 14, since the radiation source 11 is not included in a plane of opposing faces of each adjacent ones of the leaves, no X-rays from the radiation source 11 pass through the gaps between the leaves, and accordingly, there is no possibility of direct leakage of X-rays to any locations outside an exposure field. Besides, since each leaf has a profile of a rectangular parallelepiped, it can be worked readily at a low cost. On the other hand, the exposure field defined by such multi-leaf radiation shielding blocks presents such dimensional relationship in the L direction as seen in FIG. 17. In case the dimension of the exposure field in the W direction is greater at the leaf 32a than at the leaf 33a, the dimension of the exposure field in the L direction is such as indicated by La, but on the contrary in case the dimension of the exposure field in the W direction is smaller at the leaf 32a than at the leaf 33a, the dimension of the exposure field in the L direction is such as indicated by Lb, and those dimensions look different from each other. Since X-rays pass more or less obliquely through edge portions of the leaves 32a and 33a, the X-ray exposure field will be dim at portions of the dimensions La and Lb thereof. While the difference between the dimensions La and Lb is small near the center of the exposure field, it may be increased to 1 cm or so at a location of outer leaves.
On the other hand, in the case of the arrangement of FIG. 15, opposing faces of each adjacent ones of the leaves extend along an outer face of a circular cone having the apex at the radiation source 11. Such manner is illustrated in FIG. 16. Referring to FIG. 16, each of leaves has a section in an area between a generating line 61 of a face of a circular cone having the apex at the radiation source 11 and another generating line 62 of a face of another similar circular cone and makes part of a zone defined by and between the two generating lines. The leaves are defined by successive ones of faces of such similar circular cones so that they may be driven to slide along each other without mutually interfering with each other. In this instance, since opposing faces of each adjacent ones of the leaves pass the radiation source 11, if there is only a small gap between adjacent ones of the leaves, X-rays will pass through the gap and reach the patient 3 without being attenuated at all. Accordingly, X-rays may directly leak to a location outside an exposure field, which will make trouble to radiation therapy. Therefore, in the arrangement which employs such multi-leaf shielding blocks as described above, each of the leaves has a pawl 39 formed thereon so as to have such a section as shown in FIG. 15 in order to reduce or prevent such possible leakage of X-rays through gaps between adjacent ones of the leaves. In this instance, the dimension of an exposure field portion or a visually observable exposure field portion in the L direction is such as indicated by Lc in FIG. 18 when the dimension of the exposure field in the W direction is greater at the leaf 32a than at the leaf 33a, but on the contrary when the dimension of the exposure field in the W direction is smaller at the leaf 32a than at the leaf 33a, the dimension of the exposure field portion in the L direction is such as indicated by Ld in FIG. 18. Normally, the difference between the dimensions Lc and Ld is 3 to 4 mm, and since the thickness of each leaf is smaller at a portion thereof adjacent a leaf gap than at any other portion thereof, the X-ray shielding effect at such portion is lower than any other portion. Besides, the production cost of such leaves is high.
In this manner, radiation therapy is conventionally performed with a linear electron accelerator for the medical application in which such a radiation exposure field limiting apparatus of the multi-leaf type as described above is incorporated.
Since a conventional radiation exposure field limiting apparatus of the multi-leaf type is constructed in such a manner as described above, where such structure as shown in FIG. 14 is employed, the difference between the dimensions La and Lb is significantly great at an end portion of an exposure field, and a medical treatment project must necessarily be put into operation taking such difference into consideration. On the other hand, where such structure as shown in FIG. 15 is employed, since such difference between Lc and Ld exists for each leaf although it is small, it is similarly difficult to make a medical treatment project, and besides inadvertent undesirable linear leakage of X-rays to a location outside an exposure field cannot be avoided. In addition, since the profile of each leaf is complicated, the production cost is high.